At present, widely used in steering mechanisms of vehicles are hydraulically-operated power steering systems wherein a working fluid is distributed by a control valve kinematically linked with a servohydraulic actuator and a steering shaft so that during turning of the steering shaft a corresponding displacement and rotation of the control valve at an angle equal to the angle of turning of the steering shaft occurs.
However, the above construction is susceptible to fast wear and is not sufficiently reliable due to the rotation of the control valve, which affects the longevity of the steering mechanism.
One construction of power steering assembly makes use of a low speed planetary motor of the gerotor type as the servohydraulic actuator (cf., e.g., British Pat. No. 1,594,279; Cl. B 62 D 5/08, published 1981).
However, the above servohydraulic actuator is sensitive to reduced viscosity of the working fluid and impurities contained therein, which results in the loss of accuracy of the power steering assembly at increased temperature of the working fluid and a general decrease in reliability and durability.
The use of a conventional gear-type pump of external engagement makes it possible to considerably improve the characteristics of power steering.
The gear-type hydraulic pumps, however, have a number of disadvantages among which are noisy operation, increased starting torque and low efficiency at low speeds of rotation.
Kinematic linkage between the control valve of the fluid distributor and the hydraulic pump in the form of a spline shaft fails to take advantage of increased speed of rotation of the hydraulic pump, which would require less precision during the manufacture of such a pump and would result in higher efficiency.
There is also known a hydraulically-operated power steering assembly (cf. e.g., USSR Inventor's Certificate No. 770,900; Cl. B 62 D 5/06, published 1980) comprising a housing with a control valve of a fluid distributor, the central bore of the control valve accommodating a steering shaft with a screw transmission element provided thereon, and a servohydraulic actuator.
However, the power steering assembly of the above construction employs a costly plunger-type pump. Another disadvantage of the above power steering construction resides in that it lacks means for aligning the control valve.
There is further known a hydraulic power steering assembly (cf. French Pat. No. 2,052,265; Cl B 62 D 5/06, published 1971) comprising a housing of a fluid distributor with a control valve arranged inside the housing, a central bore of the control valve receiving a steering shaft provided with a screw transmission element, and a servohydraulic actuator. The steering shaft, servohydraulic actuator and the control valve are kinematically interconnected so that the control valve is capable of axial displacement when the steering shaft is turned.
During turning of the steering shaft the control valve is rotated in addition to being displaced axially, the angle of rotation of the control valve corresponding to the turning angle of the steering shaft when the vehicle wheels are steered or when the frame of a hinged-frame vehicle is collapsed to negotiate a road turn.
The aforedescribed movements of the control valve cause rather fast wear of the fluid distributor which consequently makes the power steering assembly less reliable.
In case of failure of the power assistance source, the driver applies to the steering shaft considerable torque. Because this torque is transmitted to the servohydraulic actuator through the control valve, the latter tends to deform whereby the fluid distributor may fail to result in reliable steering.
The provision of the low-speed servohydraulic pump or actuator affects proper operation of the power steering at high temperatures when the working fluid is less viscous or when this fluid contains impurities, which again affects steerability of the vehicle. In addition, high precision machining is required for fabricating low-speed servohydraulic actuators of the above construction.
One more disadvantage of the aforedescribed power steering assembly is that it employs means for aligning the control valve which is structurally complicated and difficult to assemble.